The presence of secretions, such as mucus, in the airways can trap dirt and bacteria and, if allowed to accumulate, can cause infection. Coughing is the body's natural way of a removing mucus and other secretions from the lining of the airways, and thus reduces the risk of infection.
A cough begins with a deep breath. The glottis (the opening at the top of the voice box) closes, allowing the pressure to build up in the lungs. The respiratory muscles contract, and the glottis opens, forcing air back out of the lungs.
Muscles located in the rib cage, neck and abdomen play an important role during coughing. With many neuromuscular diseases, a loss of respiratory muscle strength results in a weakened ability to cough and an increased chance of respiratory infections. This may also result from spinal cord injuries.
Mechanical devices which simulate a natural cough are known, and are generally referred to as insufflation-exsufflation devices or cough assist devices. Such devices apply a positive pressure to the patient's lungs by gradually delivering a large volume of air during an insufflation phase. Once the lungs have been expanded (similar to a normal deep breath), the device quickly reverses the flow and applies a negative pressure to extract the air volume from the lungs, and, along with it, any secretions. Cough assist devices therefore help to make the patient's cough stronger and more effective, and thus maintains clear airways, thereby reducing the chance of recurring respiratory infections. An oscillation in pressure and flow may also be superimposed during the insufflation and/or exsufflation phases to loosen secretions in the patients airways.
The valve used in a cough assist device must be capable of supplying high flow rates (in the order of a few hundred liters per minute) and must be able to quickly switch in order to reverse the flow of air.
Typically, valve design is subject to competing characteristics and thus the chosen valve design is ordinarily a compromise between these characteristics. For example, it is usually desirable to provide good sealing against fluid leakage. However, doing so typically increases friction within the valve and thus reduces the switching speed of the valve.
The invention seeks to provide a valve design which is able to achieve appropriate sealing, without adversely effecting switching speed.